A stable light source is a requirement for fiber optic testing of fiber optic cables and connections. Without a stable light source, it is impossible to ascertain whether variations arise from a tested sample or the testing light source. Factors which may produce variations in the light source include temperature, source aging effects, ambient radiant energy, and electromagnetic interference. The supply circuit of the present invention provides a stable fiber optic light source by controlling the activating current to a semi-conductor light source, such as a light-emitting diode (LED), by utilizing a feedback control system. With a feedback control system, a detector is utilized to sense the energy product of the system, and the feedback loop acts to compare the product with a desired input level and, thence, in response, to adjust input energy to a level that will produce the desired intensity of product energy.
Prior art utilizing feedback looping includes Nishizawa, et al., U.S. Pat. No. 4,329,625; and Bullock, et al., U.S. Pat. No. 4,423,478, which show light sources which sample light output to control energizing voltage.
Malissin, et al., U.S. Pat. No. 4,238,707, shows the use of a photo diode to feed back an optical signal which is combined with an input voltage in an amplifier to regulate power to a laser light source. Tanaka, et al., U.S. Pat. No. 4,467,246, relates to a light quantity controller having a feedback of a light signal combined with a voltage source in an operational amplifier to adjust power to the light source. Smith, U.S. Pat. No. 4,135,116, discloses a constant illumination control system in which an optical feedback is amplified and combined with a reference voltage in a comparator to control a light dimmer. Ferriss, et al., U.S. Pat. No. 4,431,947, discloses a light controlled source circuit in which optical feedback is combined with a voltage source to regulate light intensity.